Giải phương trình: cos 2x + cos pi/7 = 0 .

\(\cos 2x =  - \cos \frac{\pi }{7}\)\[ \Leftrightarrow \cos 2x = \cos \frac{{6\pi }}{7}\]

\[ \Leftrightarrow \left[ \begin{array}{l}2x = \frac{{6\pi }}{7} + k2\pi \\2x =  - \frac{{6\pi }}{7} + k2\pi ,\end{array} \right.\left( {k \in \mathbb{Z}} \right)\].

\[ \Leftrightarrow \left[ \begin{array}{l}x = \frac{{3\pi }}{7} + k\pi \\x =  - \frac{{3\pi }}{7} + k\pi ,\end{array} \right.\left( {k \in \mathbb{Z}} \right)\]